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    Replacing ST70 selenium diode

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    JackF999

    Posts: 13
    Join date: 2013-03-05

    Replacing ST70 selenium diode

    Post by JackF999 on Wed Mar 06, 2013 4:16 pm

    Okay, I've inspected this diode's connections.
    I've inspected the rectifier tube connections.
    I've read the yellow diode sheet.

    I's got one question.

    There are two lugs on the selenium diode.
    The one closest to the chassis has a yellow/black
    wire coming from the transformer and is very short.
    The lug farthest from the chassis has a black hookup
    wire going to the 7 lug terminal strip.

    If I want to keep this amp looking closest to stock
    and the designer's vision, what is the best way of dealing
    with these connections and placing in the new diodes.

    Bob himself in a post says to put in a 2 lug strip.
    I like this and it would look like it belongs in the amp.

    The yellow sheet says to connect the diodes at the
    rectifier tube's red wires after changing the positions.

    If I connect the diodes to the two red transformer wires
    after changing their positions...

    What do I do with the selenium diode and its two
    wire connections.

    I don't think I can just leave the yellow/black transformer
    wire hanging and I really don't know what to do about
    that other black hookup wire going from the selenium diode
    to the 7 lug terminal strip.

    Helpppp
    Thanks for reading.

    Flyquail56

    Posts: 22
    Join date: 2009-05-04

    Re: Replacing ST70 selenium diode

    Post by Flyquail56 on Thu Mar 07, 2013 12:08 am

    It sounds like you have combined two unrelated issues. The yellow sheet, as it is called, pertains to a modification to the rectifier tube socket and has nothing to do with the selenium rectifier, which supplies negative voltage to the bias supply. If you are attempting to replace the selenium rectifier, then remove the rectifier, put the 2 lug strip in its place, connect the red/black transformer wire and the banded end of a IN4007 to one lug and the black wire and the other end of the IN4007 to the other lug.

    Actually a 3 lug terminal strip might be better, because terminal strips often have one lug that is attached to the mounting leg for the strip and will therefore be grounded. You definitely don't want any of your connections here to be grounded, so be aware of this when making your connections. The only other thing to consider is the possibility of having to tweak the resistor values in the bias supply. See Bob's comments below that I have copied from an older post:

    The only minor thing that could happen is that in some amps after a change from the stock selenium rectifier to a diode the bias range will change somewhat. You won't get your "1.56 volts DC" bias voltage near the center of the bias control anymore. The standard thing to do if this does happen is to alter the values of the two 10K resistors on the 7 lug terminal strip. The usual cure is to add another 10K resistors in parallel with the 10K resistor between lugs 1 and 2 (to give a 5K effective resistance) and a larger 20K or so resistor in parallel with the 10K resistor that is now between lugs 3 and 4. A 20K resitor here will give you an effective 6.66K resistance between lugs 3 and 4. This should now give you the correct bias somewhere near the center of the bias control. The reason for the necessity of the resistor alterations is due to the greater forward flow of current by a diode as opposed to a selenium rectifier.

    JackF999

    Posts: 13
    Join date: 2013-03-05

    Thanks so much Flyquail56

    Post by JackF999 on Thu Mar 07, 2013 1:11 am

    Yikes..

    Now, after your explanation all this makes sense.
    except for one thing.....

    should the banded end go on the transformer side ( yellow/black wire )?

    Wow, this makes a great bit of difference in my thinking.

    thanks so much for your help !!!!!!!!!

    JackF999

    Posts: 13
    Join date: 2013-03-05

    Reread you post

    Post by JackF999 on Thu Mar 07, 2013 1:12 am

    So.....

    the banded end goes hard right to the yellow/black transformer
    wire and the other end goes to the smaller black hookup wire
    that connects to the 7 lug terminal strip, right ???

    Silly me.........

    Flyquail56

    Posts: 22
    Join date: 2009-05-04

    Re: Replacing ST70 selenium diode

    Post by Flyquail56 on Thu Mar 07, 2013 9:00 am

    JackF999 wrote:So.....

    the banded end goes hard right to the yellow/black transformer
    wire and the other end goes to the smaller black hookup wire
    that connects to the 7 lug terminal strip, right ???

    Silly me.........

    Yes, you are correct.

    Bob Latino
    Admin

    Posts: 1972
    Join date: 2008-11-26
    Location: Massachusetts

    Re: Replacing ST70 selenium diode

    Post by Bob Latino on Thu Mar 07, 2013 9:55 am

    That "yellow/black" wire is really a RED/BLACK wire but the dye on those cloth leads will fade with time making them appear to be "yellow/black".

    Bob

    hawaii.ken

    Posts: 157
    Join date: 2012-01-31

    Re: Replacing ST70 selenium diode

    Post by hawaii.ken on Fri Mar 08, 2013 12:35 am

    Bob is right:


      Current date/time is Tue Nov 25, 2014 9:16 pm